Lawesson's reagent (LR) is a
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
used in
organic synthesis
Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...
as a
thiation agent. Lawesson's reagent was first made popular by
Sven-Olov Lawesson, who did not, however, invent it. Lawesson's reagent was first made in 1956 during a systematic study of the reactions of arenes with
P4S10.
Preparation
Lawesson's reagent is commercially available. It can also be conveniently prepared in the laboratory by heating a mixture of
anisole
Anisole, or methoxybenzene, is an organic compound with the formula . It is a colorless liquid with a smell reminiscent of anise seed, and in fact many of its derivatives are found in natural and artificial fragrances. The compound is mainly ...
with
phosphorus pentasulfide
Phosphorus pentasulfide is the inorganic compound with the formula (empirical) or ( molecular). This yellow solid is the one of two phosphorus sulfides of commercial value. Samples often appear greenish-gray due to impurities. It is soluble in ...
until the mixture is clear and no more
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...
is formed, then
recrystallized from
toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water
Water is an inorganic compound with the c ...
or
xylene.
Samples give a strong odor of hydrogen sulfide owing to partial hydrolysis. One common and effective method of destroying the foul smelling residues is to use an excess of
sodium hypochlorite
Sodium hypochlorite is an alkaline inorganic chemical compound with the formula (also written as NaClO). It is commonly known in a dilute aqueous solution as bleach or chlorine bleach. It is the sodium salt of hypochlorous acid, consisting of ...
(
chlorine bleach).
Mechanism of action
Lawesson's reagent has a four membered ring of alternating
sulfur
Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
and
phosphorus
Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...
atoms. The central phosphorus/sulfur four-membered ring dissociates to form two reactive dithiophosphine
ylides (R-PS
2). Much of the chemistry of Lawessons's reagent is in fact the chemistry of this reactive intermediate.
:
In general, the more
electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
rich a carbonyl is, the faster the carbonyl group will be converted into the corresponding thiocarbonyl by Lawesson's reagent.
Applications
The chemistry of Lawesson's reagent and related substances has been reviewed several times.
The main use of Lawesson's reagent is the thionation of carbonyl compounds. For instance, Lawesson's reagent will convert a
carbonyl
In organic chemistry, a carbonyl group is a functional group with the formula , composed of a carbon atom double bond, double-bonded to an oxygen atom, and it is divalent at the C atom. It is common to several classes of organic compounds (such a ...
into a
thiocarbonyl. Additionally, Lawesson's reagent has been used to thionate
enones,
ester
In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
s,
lactones,
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a chemical compound, compound with the general formula , where R, R', and R″ represent any group, typically organyl functional group, groups or hydrogen at ...
s,
lactam
A lactam is a Cyclic compound, cyclic amide, formally derived from an amino alkanoic acid through cyclization reactions. The term is a portmanteau of the words ''lactone'' + ''amide''.
Nomenclature
Greek_alphabet#Letters, Greek prefixes in alpha ...
s, and
quinone
The quinones are a class of organic compounds that are formally "derived from aromatic compounds benzene.html" ;"title="uch as benzene">uch as benzene or naphthalene] by conversion of an even number of –CH= groups into –C(=O)– groups with ...
s.
:
In one study, reaction of
maltol
Maltol is a naturally occurring organic compound that is used primarily as a flavor enhancer. It is found in nature in the bark of larch trees and in the needles of pine trees, and is produced during the roasting of malt (from which it gets its n ...
with LR results in a selective oxygen replacement in two positions.
:
A combination of
silver perchlorate and Lawesson's reagent is able to act as an
oxophilic Lewis acid
A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...
with the ability to catalyze the
Diels–Alder reaction of
diene
In organic chemistry, a diene ( ); also diolefin, ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nome ...
s with α,β-unsaturated
aldehyde
In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...
s.
in some cases, alcohols may be converted to thiols by treatment with Lawesson's reagent.
Lawesson's reagent reacts with sulfoxides to form thioethers.
[
]
See also
* Woollins' reagent
References
External links
* {{cite web , url = https://www.organic-chemistry.org/namedreactions/lawessons-reagent.shtm , title = Lawesson's Reagent , publisher = Organic Chemistry Portal , accessdate = 2007-10-16
Reagents for organic chemistry
Sulfur heterocycles
Phosphorus heterocycles
Four-membered rings
Foul-smelling chemicals
Heterocyclic compounds with 1 ring
4-Methoxyphenyl compounds